[BUGFIX] Allow access to filemount directories

Currently a backend user can only access files within filemounts
but not the filemount directory itself.

After that fix LocalDriver::isWithin returns true, if the folder
itself is requested.

Resolves: #55416
Releases: 6.2
Change-Id: I455664359ec620c413bfc01787bda8766462fbab
Reviewed-on: https://review.typo3.org/27120
Reviewed-by: Ingo Schmitt
Tested-by: Ingo Schmitt
Reviewed-by: Wouter Wolters
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter